REVIEW ARTICLE ON IN SITU GEL FORMING FOR OCCULAR DRUG DELIVERY SYSTEM

ABSTRACT:
Occular drug delivery is one of the most interesting and challenging endeavors facing the pharmaceutical scientists, the major problem encountered to pharmaceutical scientist is rapid precorneal elimination of the drug, resulting in poor bioavailability and therapeutic response, because of high tear fluid turnover and dynamics. In situ-forming gels are liquid upon instillation and undergo phase transition in the ocular cul de-sac to form visco-elastic gel and these gels provides a response to environmental changes. In the past few years, an impressive number of novel temperature, pH, and ion induced in situ-forming systems have been reported for sustained ophthalmic drug delivery. Each system has its own advantages and drawbacks. The choice of a particular gel depends on its intrinsic properties and envisaged therapeutic use. This review includes various temperature, pH, and ion induced in situ-forming polymeric systems used to achieve prolonged contact time of drugs with the cornea and increase their bioavailability Now a days in situ gel have been used as vehicles for the delivery of drugs for both local treatment and systemic effects. Different administration routes other than ocular have been explored, and these cutaneous and subcutaneous delivery, dental, buccal delivery and delivery to the esophagus, stomach, colon, rectum and vagina.

Reference Id: PHARMATUTOR-ART-1313

INTRODUCTION:
The main aim of pharmacotherapeutics is the attainment of effective drug concentration at the intended site of action for a sufficient period of time to elicit the response. A major problem being faced in ocular therapeutics is the attainment of optimal concentration the site of action. Poor bioavailability of drugs from ocular dosage forms is mainly due to the tear production, transient residence time, and impermeability of corneal epithelium.1

The poor bioavailability and therapeutic response exhibited by conventional ophthalmic solutions due to rapid precorneal elimination of the drug may be overcome by the use of a gel system that are instilled as drops into the eye and undergo a sol-gel transition from the instilled dose. 2

The following characteristics are required to optimize ocular drug delivery systems.3
* A good corneal penetration.
* A prolonged contact time with corneal tissue.
* Simplicity of installation for the patient.
* A non-irritative and comfortable form (the viscous solution should not provoke lachrymation and reflex blinking)

Initial attempts to overcome the poor bioavailability of topically instilled drugs typically involved the use of ointments based on mixtures of white petrolatum and mineral oils and suspensions.4, 5 Ointments ensure superior drug bioavailability by increasing the contact time with the eye, minimizing the dilution by tears, and resisting nasolachrymal drainage because these vehicles have the major disadvantage of providing blurred vision, they are nowadays mainly used for either night time administration or for treatment on the outside and edges of the eyelids.6 Use of suspensions as ophthalmic delivery systems relies on the assumption that particles may persist in the conjunctival sac. The efficiency of suspensions has shown high variability, which occurred as a result of inadequate dosing, probably mainly due to the lack of patients compliance in adequately shaking the suspension before administration.7

These disadvantages have led to other approaches being investigated. One of the common methods to optimize prolonged precorneal residence time is to use hydrogels, liposomes, micro and nano carrier systems. In comparison with traditional formulations, these systems have the advantages of 8, 9 10.11

Even though various drug delivery systems mentioned above offer a numerous advantages over conventional drug therapy but still they are not devoid of pitfalls, including

* Poor patient compliance and difficulty of insertion as in ocular inserts.
* Tissue irritation and damage caused by penetration enhancers and collagen shields.
* Toxicity caused by insertion of foreign substances like albumin and poly butyl cyano acrylate, as in case of nano particles and microspheres.
* Change in pharmacokinetic and pharmacodynamics of the drug as caused by altering the chemical structure of the drug (prodrug approach)

HYDROGEL:
The most common way to improve drug retention on the corneal surface is undoubtedly by using polymers to increase solution viscosity. Hydrogels are polymers endowed with an ability to swell in water or aqueous solvents and induce a liquid–gel transition. Currently, two groups of hydrogels are distinguished, namely preformed and in situ forming gels. Preformed hydrogels can be defined as simple viscous solutions which do not undergo any modifications after administration. In situ forming gels are formulations, applied as solutions, sols, or Suspensions, that undergo gelation after instillation due to physicochemical, changes inherent to the eye. The objective of this review is to describe the various temperature, pH, and ion induced, in situ-forming polymeric systems used to achieve prolonged contact time of drugs with the cornea and increase their bioavailability.13, 14

Currently two groups of hydrogels are distinguished (Figure 2), namely preformed and in situ forming gels. Preformed hydrogels can be defined as simple viscous solutions, which do not undergo any modifications after administration, while in situ forming gels are formulations, applied as a solution, which undergoes gelation after instillation due to physico-chemical changes inherent to the eye.